A silver paste for power amplifiers and its preparation method

Abstract

This invention belongs to the field of packaging technology, and particularly relates to a silver paste for power amplifiers and its preparation method. The steps include: dissolving a silver source material in deionized water to obtain a silver source solution; adding an alkaline solution dropwise; stirring and reacting; separating to obtain a silver oxide precipitate; adding the silver oxide precipitate to a long-chain alcohol solvent; sieving to obtain modified silver oxide active powder; adding a polyol solvent and resin to a reaction vessel; heating and stirring; adding a functional additive; continuing to stir and disperse; and cooling to obtain a premixed solution, wherein the functional additive includes a coordinating agent; mixing the premixed solution and a silver complex solution; continuing to add the modified silver oxide active powder; and heating and stirring to obtain a silver paste for power amplifiers. The overall preparation steps feature complementary functions, thereby solving the problem that traditional silver pastes cannot achieve dense silver formation.

Description

Technical Field

[0001] This invention belongs to the field of packaging materials technology, and particularly relates to a silver paste for power amplifiers and its preparation method. Background Technology

[0002] Chip-level silver paste for power amplifiers is a critical welding material used to achieve conductive and thermally conductive connections between power amplifier chips (including gallium nitride, gallium arsenide, silicon carbide, and high-power silicon chips) and metal or ceramic substrates. Unlike ordinary silver paste used in traditional electronic packaging, chip-level silver paste must simultaneously meet multiple requirements such as ultra-high thermal conductivity, low thermal resistance, high bonding strength, and resistance to high-temperature thermal cycling to ensure the continuous and stable operation of the power amplifier under high current, wide frequency, and high heat density conditions. This type of silver paste is typically composed of modified silver oxide powder, silver complexes with low-temperature reducibility, polyol solvents, resin matrix, and functional additives such as thixotropic agents and stabilizers. Through a synergistic mechanism of solid-phase reduction and liquid-phase reduction, a dense and continuous silver interconnect network is formed at a relatively low curing temperature, allowing heat from the back of the chip to be quickly transferred to the heat dissipation substrate, thereby significantly improving the device's heat dissipation capacity and power reliability. Chip-grade silver paste is widely used in fields such as radio frequency power amplifiers, radar modules, high-frequency base stations, and high-power modules for electric vehicles because it can form a metal-like weld structure during the curing process. It is one of the core materials supporting the development of high-power electronic packaging technology.

[0003] Traditional silver paste systems typically rely on a single-path solid-phase silver source (such as silver powder or silver oxide powder) for reduction to silver. However, unavoidable gaps often exist between solid particles, resulting in numerous micropores, interfacial voids, or discontinuous regions in the cured silver layer. This leads to decreased thermal conductivity and increased interfacial thermal resistance, thus limiting the stable operation of power amplifiers under high heat flux densities. Furthermore, when using only a solid-phase silver source, its surface energy, particle dispersibility, and reactivity are limited, and coarse silver particles are easily generated during the reduction process, further hindering the formation of a dense silver network. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a silver paste for power amplifiers and its preparation method, which aims to solve the problem that existing power amplifier chip-level silver pastes are difficult to form a dense and continuous silver interconnect layer during low-temperature curing.

[0005] To solve the above-mentioned technical problems, the present invention provides a method for preparing silver paste for power amplifiers, comprising the following steps:

[0006] S1. Dissolve the silver source material in deionized water to obtain a silver source solution, add an alkaline solution dropwise, stir to react, separate to obtain silver oxide precipitate, add the silver oxide precipitate to a long-chain alcohol solvent, and sieve to obtain modified silver oxide active powder.

[0007] S2. Add polyol solvent and resin to a reaction vessel, heat and stir, add functional additives, continue stirring and dispersing and cool to obtain a premixed liquid, wherein the functional additives include a complexing agent;

[0008] S3. Mix the premixed solution and the silver complex solution, add the modified silver oxide active powder, heat and stir to obtain silver paste for power amplifiers.

[0009] In some embodiments, step S1 includes:

[0010] S1.1 Add the silver source material to a predetermined volume of deionized water and magnetically stir at 300-500 rpm for 10-20 minutes at 20-30°C to obtain a silver source solution.

[0011] S1.2. While maintaining stirring, add the alkaline solution dropwise to the silver source solution at a rate of 2-5 mL / min until the pH of the silver source solution reaches 10-12, and continue stirring for 30-60 min. Separate the wet silver oxide precipitate by filtration.

[0012] S1.3 Add the wet silver oxide precipitate to a long-chain alcohol solvent at a solid-liquid ratio of 1:(3-8), and stir at 300-600 rpm for 30-90 min at 50-80℃ to obtain silver oxide alcohol slurry;

[0013] S1.4. The silver oxide alcohol slurry is separated by suction filtration and then vacuum dried at 40-60℃ for 2-6 hours. After pulverization and sieving, modified silver oxide active powder is obtained.

[0014] In some embodiments, in step S1, the silver source material includes at least one of silver nitrate, silver acetate, and silver lactate; the alkaline solution includes at least one of sodium hydroxide solution, potassium hydroxide solution, and ammonia water; and the long-chain alcohol solvent includes at least one of dodecanol, tetradecanol, and hexadecylol.

[0015] In some embodiments, step S2 includes:

[0016] S2.1 Mix the polyol solvent with the resin, start stirring at 20-30°C and slowly heat to 60-80°C, stir at 300-500 rpm for 30-60 minutes to obtain the resin solution;

[0017] S2.2. Under the condition of maintaining the resin solution at 60-80℃ and stirring at 300-500rpm, add the complexing agent and continue stirring for 20-40min to obtain a resin solution containing the complexing agent.

[0018] S2.3 Functional additives include thixotropic agents, dispersants, and inorganic fillers. Thixotropic agents, dispersants, and inorganic fillers are added sequentially to the resin solution containing the coordinating agent. The stirring speed is adjusted to 800-1500 rpm and dispersed for 20-40 minutes to obtain a polyol resin mixture.

[0019] S2.4. Stop heating and cool the polyol resin mixture to 20-30°C. Maintain this temperature for 10-20 minutes while stirring at 200-300 rpm. 0.06~ Vacuum degassing at 0.09 MPa for 10–20 min, then stop stirring and let stand to obtain the premixed solution.

[0020] In some embodiments, in step S2, the polyol solvent includes at least one of ethylene glycol, diethylene glycol, and triethylene glycol monobutyl ether; the resin includes at least one of acrylic resin, polyether-modified resin, and polyurethane resin; the coordinating agent includes at least one of citric acid, tartaric acid, and triethanolamine; the thixotropic agent includes at least one of fumed silica, polyamide wax, and organobentonite; the dispersant includes at least one of polycarboxylate dispersant and sulfonate dispersant; and the inorganic filler includes at least one of nano-silica, nano-alumina, and nano-zirconia.

[0021] In some embodiments, step S3 includes:

[0022] S3.1 Add the premixed solution to the reactor and start stirring at 200-400 rpm at 20-30℃. Add the silver complex solution to the premixed solution at a constant flow rate, controlling the addition time of the silver complex solution to 10-30 min. Continue stirring at 60-80℃ for 20-40 min.

[0023] S3.2 Adjust the stirring temperature to 30-50℃ and maintain the stirring speed at 300-600 rpm. Add the modified silver oxide active powder in batches, stirring for 10-20 minutes after each addition, until all the powder is added and continue stirring for 20-40 minutes to obtain the silver paste intermediate.

[0024] S3.3 Cool the silver paste intermediate to 20-30°C, and... 0.06~ Vacuum degassing at 0.09 MPa for 10–30 min, followed by 1–3 rounds of grinding to control the fineness to no more than 10 μm. After grinding, the paste is placed in a sealed container and cured at 5–25 °C for 12–48 h to obtain silver paste for power amplifiers.

[0025] In some embodiments, step S3.2 further includes adjusting the viscosity of the silver paste intermediate to 80,000–250,000 mPa·s. If the viscosity of the silver paste intermediate is less than 80,000 mPa·s, a thixotropic agent or resin is added in several batches to increase the viscosity. The amount of thixotropic agent or resin added each time is 0.1–0.5% of the total mass of the silver paste intermediate. After each addition, the mixture is stirred at a speed of 200–400 rpm for 10–20 min. If the viscosity of the silver paste intermediate is greater than 250,000 mPa·s, a polyol solvent accounting for 1–5% of the total mass of the silver paste intermediate is added in several batches. After each addition, the mixture is stirred at a speed of 200–400 rpm for 5–10 min.

[0026] In some embodiments, step S3 further includes:

[0027] The organic complexed silver source is mixed with an organic solvent and stirred at 300-500 rpm for 10-20 min at 20-30°C to obtain an organic silver salt solution. The main ligand is added and the mixture is stirred at 30-40°C for 20-40 min to obtain a primary silver complex solution.

[0028] The primary silver complex solution was maintained at 30–40°C and 300–500 rpm under stirring conditions. An auxiliary ligand was added, and the addition time was controlled to be 10–20 min. After the addition was completed, stirring was continued for 20–40 min to obtain a graded silver complex solution.

[0029] While maintaining a stirring temperature of 30–40°C, a stabilizer is added to the fractionated silver complex solution at a concentration of 0.1–3% of the total mass of the fractionated silver complex solution. The mixture is then stirred at 300–500 rpm for 20–30 minutes to obtain the silver complex solution.

[0030] In some embodiments, the organic complexed silver source includes at least one of silver gluconate, silver citrate, and silver β-hydroxybutyrate; the organic solvent includes at least one of triethylene glycol monobutyl ether, dipropylene glycol methyl ether, and polyglycerol-3; the main ligand includes at least one of tris(hydroxymethyl)aminomethane, polyglycerol-4, and N-hydroxyethylethylenediaminetriacetic acid; the auxiliary ligand includes at least one of dopamine, inositol hexaphosphate, and sodium polyglutamate; and the stabilizer includes at least one of tris(2-ethylhexyl) phosphite, polyvinylpyrrolidone, and carboxymethyl chitosan.

[0031] This invention proposes a silver paste for power amplifiers, characterized in that it is prepared by the method described above for preparing a silver paste for power amplifiers;

[0032] The steps for using silver paste for power amplifiers include:

[0033] The metal or ceramic substrate to be packaged is cleaned. The dispensing pressure, speed or stencil thickness is set according to the chip area and soldering thickness requirements. The dispensing or printing operation is completed in an environment of 20-30℃, so that the silver paste for the power amplifier is uniformly deposited on the pad area of ​​the metal or ceramic substrate, and the film thickness is controlled within the range of 20-60μm.

[0034] Before the silver paste for power amplifiers has cured, accurately mount the power amplifier chip onto the patterned area of ​​the silver paste for power amplifiers. Apply mounting pressure of 50-200g by light pressure or mounting head. After mounting, let stand for 1-3 minutes to obtain the mounted substrate.

[0035] The mounted substrate is transferred to an oven and preheated at 60–150°C for 30–60 minutes, then further heated to 180–250°C and cured in air, nitrogen, or a micro-oxygen environment for 60–90 minutes. After curing, it is cooled to below 40°C to complete the soldering. In some embodiments, the process between steps S3.3 and S3.4 further includes:

[0036] The deposited composite material was placed in a tube furnace and heated to 300–450°C at a rate of 2–5°C / min under argon protection. Then, a mixed gas containing oxygen and ammonia was introduced at a total flow rate of 50–200 mL / min for 0.5–2 h to obtain the oxygen-nitrided deposited composite material.

[0037] Compared with the prior art, the silver paste for power amplifiers and its preparation method disclosed in this invention have the following advantages:

[0038] Step S1 involves precipitating the silver source into silver oxide and then surface-modifying it with a long-chain alcohol. This results in a modified silver oxide active powder with higher dispersibility and surface reactivity, making it easier for the solid-phase silver source to distribute uniformly in the system and participate more fully in the solid-phase reduction to silver during the curing stage. Step S2 uses a polyol solvent, resin, and functional additives such as coordinating agents and thixotropic agents to form a stable organic continuous phase. This ensures both the uniform dispersion of the solid-phase silver oxide powder and the silver complex solution and provides a stable environment for the coordination structure of the silver complex, allowing the entire system to remain rheologically stable before curing and preventing premature agglomeration or sedimentation. Step S3 involves mixing the premixed liquid with the silver complex solution and then adding modified silver oxide active powder, so that the liquid-phase silver source and the solid-phase silver source can coexist in the same system. Under heating conditions, silver monomers are generated through two pathways: ligand desorption reduction and polyol reduction. The liquid-phase silver formation pathway can fill the micropores between solid-phase silver particles that could not be completely welded, so that the final solidified structure forms a dense and continuous silver network. Therefore, the technical features of the overall preparation steps form a complementary function, thereby solving the problem that traditional silver paste cannot form dense silver. Attached Figure Description

[0039] Figure 1 This is a flowchart of a method for preparing silver paste for power amplifiers according to an embodiment of the present invention. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0041] Please refer to Figure 1 This invention proposes a method for preparing silver paste for power amplifiers, comprising the following steps:

[0042] S1. Dissolve the silver source material in deionized water to obtain a silver source solution. Add an alkaline solution dropwise, stir and react, and separate to obtain silver oxide precipitate. Add the silver oxide precipitate to a long-chain alcohol solvent, and sieve to obtain modified silver oxide active powder. The silver source material includes at least one of silver nitrate, silver acetate, and silver lactate. The alkaline solution includes at least one of sodium hydroxide solution, potassium hydroxide solution, and ammonia water. The long-chain alcohol solvent includes at least one of dodecanol, tetradecanol, and hexadecylol.

[0043] Step S1 includes:

[0044] S1.1 Add the silver source material to a predetermined volume of deionized water and magnetically stir at 300-500 rpm for 10-20 minutes at 20-30°C to obtain a silver source solution.

[0045] By dissolving the silver source material in deionized water at low temperature and dispersing it uniformly with moderate stirring, silver ions can be fully dissociated in the aqueous phase, forming a stable and homogeneous silver ion solution. Silver nitrate, silver acetate, and silver lactate all have good solubility in water. Among them, nitrate, acetate, and lactate ions have weak coordination characteristics, which promote the existence of silver in a free ionic state. This step ensures that silver ions can quickly and fully participate in the precipitation reaction when alkaline solution is added subsequently, resulting in uniform silver oxide particles and avoiding the formation of coarse particles due to local supersaturation caused by insufficient dissolution. This lays the foundation for the formation of a fine-grained, highly reactive silver oxide precursor.

[0046] S1.2. While maintaining stirring, add the alkaline solution dropwise to the silver source solution at a rate of 2-5 mL / min until the pH of the silver source solution reaches 10-12, and continue stirring for 30-60 min. Separate the wet silver oxide precipitate by vacuum filtration.

[0047] By adding an alkaline solution at a constant, low dropping rate, the pH of the solution gradually increases within a controllable range, which helps silver ions to combine with hydroxide ions in a stable manner and precipitate as silver oxide without instantaneous local supersaturation. Sodium hydroxide, potassium hydroxide, and ammonia can all serve as effective alkaline sources. Hydroxide ions react with silver ions to form silver oxide precipitate, while the silver-ammonia complex formed by ammonia helps regulate the precipitation rate, resulting in finer silver oxide particles. Extending the stirring time promotes crystal aging, making the silver oxide structure more stable and uniform. This step yields wet silver oxide with a concentrated particle size distribution and high surface energy, providing a highly reactive solid-phase precursor for subsequent long-chain alcohol modification reactions and improving the sintering density of the final silver paste.

[0048] S1.3 Add the wet silver oxide precipitate to a long-chain alcohol solvent at a solid-liquid ratio of 1:(3-8), and stir at 300-600 rpm for 30-90 min at 50-80℃ to obtain silver oxide alcohol slurry.

[0049] By adding wet silver oxide precipitate to a long-chain alcohol solvent and dispersing it thoroughly under heating and stirring conditions, the surface of the silver oxide particles is coated with long-chain fatty alcohols such as dodecanol, tetradecanol, or hexadecylol. Long-chain alcohol molecules possess hydrophobic carbon chains and polar hydroxyl groups. Their polar ends can form hydrogen bonds or weak coordination with the hydroxyl groups or oxidation sites on the silver oxide surface, while the hydrophobic chain portion forms an ordered organic shell. This shell not only improves the dispersibility of silver oxide in the organic system but also enhances the wettability of the particle surface during subsequent sintering, making it easier to fuse with the liquid silver source and form a uniform and dense silver network. Simultaneously, the modified silver oxide exhibits enhanced activity, resulting in smoother thermal reduction and thus improving the reaction efficiency of the solid-phase silver formation pathway.

[0050] S1.4. The silver oxide alcohol slurry is separated by suction filtration and then vacuum dried at 40-60℃ for 2-6 hours. After pulverization and sieving, modified silver oxide active powder is obtained.

[0051] By filtering, vacuum drying, and pulverizing and sieving the silver oxide alcohol slurry, the long-chain alcohol coating structure is stably solidified on the surface of the silver oxide particles, while free solvent and some adsorbed moisture are removed. Vacuum drying reduces the probability of agglomeration, ensuring good dispersion of the particles during the drying process; while the pulverizing and sieving steps ensure that the final modified silver oxide powder has uniform particle size and good flowability. The surface organic shell gives the particles good interfacial compatibility, allowing them to be uniformly dispersed in the polyol-resin system of the silver paste. During the curing process, the particles synergistically react with the silver monomers released from the silver complex to form a dual-pathway silver formation structure in which solid-phase framework silver and liquid-phase nano-silver coexist, thereby improving the thermal conductivity, density, and power cycle reliability of the final silver paste.

[0052] S2. Add polyol solvent and resin to a reaction vessel, heat and stir, add functional additives, continue stirring and dispersing and cool to obtain a premixed liquid, wherein the functional additives include a complexing agent;

[0053] Step S2 includes:

[0054] S2.1 Mix the polyol solvent with the resin, start stirring at 20-30°C and slowly heat to 60-80°C, stir at 300-500 rpm for 30-60 min to obtain a resin solution; the polyol solvent includes at least one of ethylene glycol, diethylene glycol, and triethylene glycol monobutyl ether, and the resin includes at least one of acrylic resin, polyether modified resin, and polyurethane resin.

[0055] By mixing polyol solvents with resins at low temperatures and then fully dissolving them under heating and stirring conditions, a stable and homogeneous continuous phase structure is formed in the polyol system. Ethylene glycol, diethylene glycol, and triethylene glycol monobutyl ether are all polar solvents with strong hydroxyl-hydroxyl or hydroxyl-ether oxygen interactions, which can effectively dissolve acrylic resins, polyether-modified resins, and polyurethane resins, allowing the long-chain molecules of the resin to fully unfold and uniformly disperse in the solvent system. This step significantly reduces the initial viscosity of the system, giving the resulting resin solution good flowability and wettability, providing a stable carrier for the subsequent dispersion of ligands, thixotropic agents, and inorganic fillers. Furthermore, the polyols will also act as potential reducing components during the subsequent curing process, undergoing a synergistic reaction with the silver source. Therefore, establishing a homogeneous organic continuous phase structure in this step is crucial for achieving the printability and dense curing of the silver paste.

[0056] S2.2. Under the condition of maintaining the resin solution at 60-80℃ and stirring at 300-500rpm, add the complexing agent and continue stirring for 20-40min to obtain a resin solution containing the complexing agent; the complexing agent includes at least one of citric acid, tartaric acid, and triethanolamine.

[0057] A ligand is added to the resin solution, and the reaction continues under stable stirring conditions. This allows the carboxyl, hydroxyl, or nitrogen groups in the ligand to form hydrogen bonds, ionic bonds, or weak coordination interactions with the polar functional groups on the resin chain segments, thereby constructing a molecular-level coordination environment capable of adsorbing silver ions. Citric acid and tartaric acid, with their multi-carboxyl structures, can provide cooperative coordination sites for silver ions in the subsequent silver complex solution; triethanolamine has a strong nitrogen coordination ability, which can effectively improve the metal ion binding capacity of the system. This step enables the resin system to pre-conjugate metal ion complexes, which helps to regulate the stability of the silver complex in the organic phase during subsequent mixing steps, preventing premature reduction of silver ions due to localized concentration. Simultaneously, the ligand's effect on adjusting the resin polarity distribution improves the compatibility between the filler and the resin interface, resulting in more stable rheological properties in the final premixed solution system.

[0058] S2.3 The functional additives include thixotropic agents, dispersants, and inorganic fillers. Thixotropic agents, dispersants, and inorganic fillers are added sequentially to the resin solution containing the coordinating agent. The stirring speed is adjusted to 800-1500 rpm and dispersed for 20-40 minutes to obtain a polyol resin mixture. The thixotropic agent includes at least one of fumed silica, polyamide wax, and organobentonite. The dispersant includes at least one of polycarboxylate dispersant and sulfonate dispersant. The inorganic filler includes at least one of nano silica, nano alumina, and nano zirconium oxide.

[0059] Sequentially adding thixotropic agents, dispersants, and inorganic fillers to a resin solution and dispersing them under high shear conditions effectively constructs the three-dimensional thixotropic network structure required for silver paste systems. Thixotropic agents, such as fumed silica, polyamide wax, and organobentonite, can form a spatial network connected by hydrogen bonds or van der Waals forces, maintaining a high yield stress at rest and thus preventing silver powder sedimentation. During shearing, the network structure breaks down, exhibiting excellent shear-thinning properties, which is beneficial for printing or dispensing operations. Dispersants, such as polycarboxylate dispersants and sulfonate dispersants, can improve the dispersion stability of fillers through electrostatic repulsion and steric hindrance, preventing the agglomeration of nano-silica, nano-alumina, or nano-zirconia. High-shear mixing allows for uniform bonding between the resin and filler interfaces, forming a stable composite rheological structure. This ensures that the system maintains good shape retention and uniformity when silver sources are subsequently added, contributing to improved printing accuracy and anti-sagging ability of the silver paste.

[0060] S2.4. Stop heating and cool the polyol resin mixture to 20-30°C. Maintain this temperature for 10-20 minutes while stirring at 200-300 rpm. 0.06~ Vacuum degassing at 0.09 MPa for 10–20 min, then stop stirring and let stand to obtain the premixed solution.

[0061] By stopping heating and gradually cooling the system, the viscosity recovers and stabilizes, facilitating the reconstruction of the thixotropic network structure and the rearrangement of resin segments, thus achieving stable rheological behavior. During the holding phase at a low stirring rate, microbubbles in the system gradually migrate to the liquid surface. Residual bubbles are then completely removed through a degassing process under vacuum, significantly reducing the risk of void formation due to bubble expansion during subsequent silver paste curing. The final settling step allows the system to reach rheological and interfacial thermodynamic equilibrium, forming a uniform and stable premix, providing a good dispersion basis for the subsequent addition of silver complex solution and modified silver oxide active powder. This step not only improves the overall uniformity of the silver paste but also lays the foundation for achieving a dense silver interconnect structure.

[0062] S3. Mix the premixed solution and the silver complex solution, add the modified silver oxide active powder, heat and stir to obtain silver paste for power amplifiers.

[0063] Step S3 includes:

[0064] An organic silver complex source is mixed with an organic solvent and stirred at 300-500 rpm for 10-20 minutes at 20-30°C to obtain a silver organic salt solution. A main ligand is added, and the mixture is stirred at 30-40°C for another 20-40 minutes to obtain a primary silver complex solution. The organic silver complex source includes at least one of silver gluconate, silver citrate, and silver β-hydroxybutyrate. The organic solvent includes at least one of triethylene glycol monobutyl ether, dipropylene glycol methyl ether, and polyglycerol-3. The main ligand includes at least one of tris(hydroxymethyl)aminomethane, polyglycerol-4, and N-hydroxyethylethylenediaminetriacetic acid.

[0065] By dissolving organic silver complex sources in triethylene glycol monobutyl ether, dipropylene glycol methyl ether, or polyglycerol polyol solvents and uniformly stirring at low temperature, organic silver salts such as silver gluconate, silver citrate, or silver β-hydroxybutyrate can be fully dissociated and form a stable organic silver salt dispersion system. The anions of these organic silver salts contain polyhydroxy or polycarboxyl groups, which can form hydrogen bonds or weak coordination interactions with the polyol solvent, keeping silver ions dispersed in the solvent without precipitation or local aggregation. This step establishes a uniform initial distribution of silver ions in the organic phase, laying the foundation for the subsequent construction of complex structures with controllable reduction pathways. It also significantly improves the compatibility of the silver source in the resin system, avoiding the phase separation and premature reduction problems associated with traditional aqueous silver sources. After adding tris(hydroxymethyl)aminomethane, polyglycerol, or polycarboxyl ligands, the main coordinating agent can utilize its hydroxyl, amino, or polydentate coordination structures to form stable monovalent or polyvalent coordination complexes with silver ions, transforming the silver ions from a free state into a complex state with a defined coordination structure. This complexed state significantly enhances the thermal stability of silver ions, making them less prone to premature reduction at room temperature. Simultaneously, it enables hierarchical desorption during low-temperature curing, thereby regulating the nucleation rate and deposition behavior of silver atoms during the curing process. The primary coordination structure constructed in this step can be considered the core foundation of the subsequent low-temperature silver formation mechanism, enabling this invention to form a dense and uniform nano-silver network at low temperatures.

[0066] The primary silver complex solution was maintained at 30–40°C and 300–500 rpm under stirring conditions. An auxiliary ligand was added, and the addition time was controlled to be 10–20 min. After the addition was completed, stirring was continued for 20–40 min to obtain a graded silver complex solution. The auxiliary ligand included at least one of dopamine, inositol hexaphosphate, and sodium polyglutamate.

[0067] Auxiliary ligands such as dopamine, inositol hexaphosphate, and sodium polyglutamate possess stronger electron-donating capabilities, enabling the formation of a second coordination shell structure around silver ions. This creates a hierarchical complexation system of main and auxiliary ligands, further enhancing the coordination stability of silver ions. This bilayer coordination structure endows silver complexes with two key advantages: firstly, it significantly improves storage stability at room temperature, preventing premature reduction of silver ions due to thermal disturbances or fluctuations in the resin environment; secondly, during the curing stage, the desorption temperature of the auxiliary ligands is lower than that of the main ligand, allowing silver ions to be released gradually according to a thermal program, thereby controlling the nucleation density and growth rate of silver nanoparticles.

[0068] While maintaining a stirring temperature of 30–40°C, a stabilizer is added to the fractionated silver complex solution at a concentration of 0.1–3% of the total mass of the solution. The mixture is then stirred at 300–500 rpm for 20–30 minutes to obtain the silver complex solution. The stabilizer includes at least one of tris(2-ethylhexyl) phosphite, polyvinylpyrrolidone, and carboxymethyl chitosan.

[0069] Adding stabilizers such as tris(2-ethylhexyl) phosphite, polyvinylpyrrolidone, or carboxymethyl chitosan to the graded complex can further form an adsorption layer or steric hindrance layer, preventing aggregation or spontaneous reduction between silver complexes. The introduction of stabilizers not only improves the storage stability of the system but also participates in auxiliary regulation during the curing process, allowing silver ions to be released and deposited more uniformly in the resin system. In particular, phosphite stabilizers, which have weak reducing properties during heating, can promote the formation of silver monomers at suitable temperatures, creating a synergistic effect between the liquid-phase silver formation pathway and the solid-phase silver oxide reduction pathway. This step enables the silver complex system to possess controllable and predictable thermal reduction behavior, ensuring that the final silver paste cures to form a dense and continuous silver interconnect structure.

[0070] S3.1 Add the premixed solution to the reactor and start stirring at 200-400 rpm at 20-30℃. Add the silver complex solution to the premixed solution at a constant flow rate, controlling the addition time of the silver complex solution to be 10-30 min. Continue stirring at 60-80℃ for 20-40 min.

[0071] Adding the silver complex solution to the premix at a constant flow rate and mixing thoroughly at a low rotation speed helps to ensure that the silver ions in the silver complex are evenly distributed in the resin-polyol continuous phase, preventing localized excessive concentrations or loss of reducibility due to resin phase coating. Subsequently, heating the system to 60-80 degrees Celsius promotes the desorption of some weakly coordinated auxiliary ligands, bringing the silver complex into a reducible pre-activated state. This also enhances the resin's wetting ability of the silver complex, ensuring good interfacial contact between the liquid-phase silver source and the subsequently added modified silver oxide active powder. This step ensures that the silver paste system can simultaneously undergo the synergistic reaction pathways of liquid-phase nano-silver generation and solid-phase silver oxide reduction during the curing stage, thereby improving the density and continuity of the final silver layer.

[0072] S3.2 Adjust the stirring temperature to 30-50℃ and maintain the stirring speed at 300-600 rpm. Add the modified silver oxide active powder in batches, stirring for 10-20 minutes after each addition, until all the powder is added and continue stirring for 20-40 minutes to obtain the silver paste intermediate.

[0073] Step S3.2 further includes adjusting the viscosity of the silver paste intermediate to 80,000–250,000 mPa·s. If the viscosity of the silver paste intermediate is less than 80,000 mPa·s, thixotropic agent or resin is added in several batches to increase the viscosity. The amount of thixotropic agent or resin added each time is 0.1–0.5% of the total mass of the silver paste intermediate. After each addition, the mixture is stirred at 200–400 rpm for 10–20 min. If the viscosity of the silver paste intermediate is greater than 250,000 mPa·s, polyol solvent accounting for 1–5% of the total mass of the silver paste intermediate is added in several batches. After each addition, the mixture is stirred at 200–400 rpm for 5–10 min.

[0074] Modified silver oxide active powder is added in batches at 30-50 degrees Celsius to ensure thorough integration with the resin-polyol system and silver complex, guaranteeing uniform dispersion of solid-phase silver oxide within the system. Because the modified silver oxide is coated with long-chain alcohol molecules, its interfacial compatibility is superior to unmodified silver oxide, facilitating the formation of a stable suspension structure and making it easier to be reduced to metallic silver by the polyol during curing. This process establishes the reaction basis for the solid-phase silver formation pathway, resulting in a large-particle silver framework after curing and enhancing the thermal conductivity of the silver layer.

[0075] Subsequently, viscosity adjustment is performed to control the final viscosity of the system within the range of 80,000 to 250,000 millipascals per second, ensuring that the silver paste has appropriate flowability and shape retention during printing or dispensing. If the viscosity is too low, the system's yield stress can be increased by adding thixotropic agents or resins in stages, preventing silver powder sedimentation or pattern diffusion; if the viscosity is too high, the system's flow resistance can be reduced by adding polyol solvents, improving printability. The viscosity adjustment process allows the system to simultaneously meet the requirements of processability and structural stability, thereby obtaining a silver paste intermediate with controllable performance.

[0076] S3.3 Cool the silver paste intermediate to 20-30°C, and... 0.06~ Vacuum degassing at 0.09 MPa for 10–30 min, followed by 1–3 rounds of grinding to control the fineness to no more than 10 μm. After grinding, the paste is placed in a sealed container and cured at 5–25 °C for 12–48 h to obtain silver paste for power amplifiers.

[0077] By cooling the silver paste intermediate to 20-30 degrees Celsius and performing vacuum degassing, microbubbles generated during mixing, dispersion, or desorption of silver complexes can be effectively removed from the system, thus preventing the formation of macroscopic pores or interfacial voids after curing. Subsequent one to three rounds of grinding thoroughly break down large agglomerates and increase the interfacial contact area between the silver powder and resin, controlling the fineness to below ten micrometers. This helps improve the uniformity of the silver paste and the density of the cured silver layer. Finally, static curing stabilizes the interfacial structure between the coordinating agent, polyol, resin, and silver source, ensuring stable viscosity and preventing delamination or hardening of the silver paste during storage. This step ensures that the final silver paste exhibits excellent thermal conductivity, mounting reliability, and long-term thermal cycling stability in power amplifier packaging.

[0078] Example 1:

[0079] In step S1.1, 100 parts by mass of silver nitrate, the silver source material, are added to 500 parts by mass of deionized water and magnetically stirred at 300 rpm for 15 min at 20-25°C to obtain a transparent silver source solution. In step S1.2, a 1 mol / L sodium hydroxide solution is prepared as an alkaline solution and added dropwise to the silver source solution at a rate of 3 mL / min while maintaining stirring. When the pH rises to 11, the dropwise addition is stopped and stirring continues for 40 min. The wet silver oxide precipitate is obtained by filtration. In step S1.3, the wet silver oxide precipitate is added to the long-chain alcohol solvent dodecanol at a solid-liquid ratio of 1:5 and stirred at 400 rpm for 60 min at 70°C to obtain a homogeneous silver oxide alcohol slurry. In step S1.4, the silver oxide alcohol slurry is filtered, and the filter cake is heated at 50°C. Vacuum drying at 0.08 MPa for 4 hours, followed by mechanical pulverization and passing through a 300-mesh sieve, yielded modified silver oxide active powder with a particle size D50 of approximately 1.5 μm. In step S2, 30 parts by weight of the polyol solvent triethylene glycol monobutyl ether and 20 parts by weight of acrylic resin were added to a reaction vessel. Stirring was started at 25°C and the temperature was slowly increased to 70°C. Stirring was continued at 400 rpm for 40 minutes to obtain a transparent resin solution. At this temperature and stirring conditions, 2 parts by weight of the ligand citric acid were added, and stirring continued for 30 minutes to obtain a resin solution containing the citric acid ligand. Subsequently, 3 parts by weight of the thixotropic agent fumed silica, 1 part by weight of the polycarboxylate dispersant, and 5 parts by weight of nano-silica were added sequentially. The stirring speed was increased to 1000 rpm and dispersed for 30 minutes to obtain a polyol resin mixture. Heating was stopped, and the mixture was allowed to cool naturally to 25°C. After holding at this temperature for 15 minutes with stirring at 250 rpm, the mixture was then... Vacuum degassing was performed at 0.08 MPa for 15 min, stirring was stopped, and the mixture was allowed to stand to obtain a homogeneous premix. In step S3, the silver complex solution was prepared: 20 parts by mass of the organic silver complex source silver gluconate were dissolved in a mixed organic solvent of 40 parts by mass of triethylene glycol monobutyl ether and 310 parts by mass of polyglycerol-3. The mixture was stirred at 400 rpm for 15 min at 25 °C to obtain a silver organic salt solution. Then, 5 parts by mass of tris(hydroxymethyl)aminomethane were added, and stirring was continued at 30–35 °C for 30 min to obtain a primary silver complex solution. 1 part by mass of the auxiliary ligand dopamine was added at 30–35 °C and 300 rpm, and the addition was completed within 10 min, followed by stirring for another 30 min to obtain a graded silver complex solution. Then, 2 parts by mass of the stabilizer tris(2-ethylhexyl) phosphite were added under the same temperature and stirring conditions, and the mixture was stirred for 25 min to obtain a stable silver complex solution. Then, proceed to step S3.1, adding 100 parts by mass of the premixed solution obtained in step S2 to the reactor. Stirring is started at 25°C and 300 rpm. 40 parts by mass of the silver complex solution are added at a constant flow rate over 20 minutes. The temperature is raised to 70°C and stirring continues for 30 minutes. The stirring temperature is adjusted to 30–50°C, maintaining a speed of 300–600 rpm. Before adding the modified silver oxide active powder in batches, a nucleation regulator is added. The nucleation regulator includes at least one of citric acid, tartaric acid, and ethylene glycol, and the nucleation regulator accounts for a certain percentage of the total mass. The silver paste intermediate, estimated at 2% of its total mass, was stirred at 40°C for 10 minutes to ensure uniform distribution of the nucleation regulator within the resin-polyol continuous phase. Subsequently, modified silver oxide active powder was added in batches, with diethylene glycol, an auxiliary wetting agent, added simultaneously with each batch. The nucleation regulator comprised 3% of the total mass of the silver paste intermediate. Each batch was stirred for 20 minutes after addition, and after all batches were added, stirring continued at 50°C for 40 minutes to allow the nucleation regulator and polyol to jointly regulate the nucleation behavior between the silver complex and the modified silver oxide, ultimately yielding the silver paste intermediate. The measured viscosity was 150,000 mPa·s, falling within the range of 80,000–250,000 mPa·s, requiring no further adjustment. In step S3.3, the silver paste intermediate was cooled to 25°C. Vacuum degassing was performed for 20 minutes under 0.08 MPa, followed by grinding twice on a three-roll mill to control the fineness to no more than 8 μm. Finally, the silver paste was placed in a sealed container and allowed to stand at 10°C for 24 hours to obtain the silver paste for power amplifiers in Example 1.

[0080] Example 2:

[0081] In step S1, 100 parts by mass of silver acetate were selected as the silver source material, and other conditions were the same as in Example 1. The final product was a modified silver oxide active powder with a particle size D50 of approximately 1.8 μm. The long-chain alcohol solvent was replaced with tetradecyl alcohol. In step S2, a combination of 20 parts by mass of diethylene glycol and 10 parts by mass of triethylene glycol monobutyl ether was used as the polyol solvent. 25 parts by mass of polyether-modified resin were used as the resin. The mixture was stirred for 45 min under the same heating conditions to obtain a resin solution. 3 parts by mass of tartaric acid were used as the coordination agent. The mixture was stirred at 60–80 °C for 30 min to enhance the polycarboxyl coordination ability of the resin solution. 4 parts by mass of polyamide wax were used as the thixotropic agent, 1 part by mass of sulfonate dispersant was used as the dispersant, and a combination of 4 parts by mass of nano-alumina and 3 parts by mass of nano-zirconia was used as the inorganic filler. The mixture was dispersed at 1200 rpm for 30 min to obtain a polyol resin mixture. After cooling and vacuum degassing for 15 min, a premix was obtained. In step S3, the organic silver complex source is silver citrate (25 parts by mass), the organic solvent is triethylene glycol monobutyl ether (30 parts by mass) and polyglycerol (315 parts by mass), and the main ligand is polyglycerol (46 parts by mass). Under the same conditions, a primary silver complex solution is obtained. The auxiliary ligand is inositol hexaphosphate (2 parts by mass), and the stabilizer is polyvinylpyrrolidone (3 parts by mass). Finally, a silver complex solution with a viscosity of 1500 mPa·s is obtained. The stirring temperature was adjusted to 50℃ and the stirring speed was maintained at 600 rpm. Before adding the modified silver oxide active powder in batches, an interface regulator was added. The interface regulator included at least one of γ-glycidoxypropyltrimethoxysilane and sulfonate-type dispersant enhancer. The interface regulator accounted for 1% of the expected total mass of the silver paste intermediate. The mixture was stirred at 40℃ for 10 min to allow the interface regulator to form an adsorption pre-film in the resin-polyol continuous phase. Subsequently, the modified silver oxide active powder was added in batches, and stirred for 20 min after each batch to allow the silane and dispersant enhancer to synergistically form a stable wetting layer on the silver oxide surface. After all the powder was added, stirring was continued for 40 min to ensure that the powder was uniformly dispersed in the resin continuous phase, thus obtaining the silver paste intermediate. The viscosity of the silver paste intermediate was measured to be slightly lower than 80,000 mPa·s. Therefore, 0.3% by mass of polyamide wax and polyether modified resin were added in two batches, and stirred for 15 min after each addition. The final viscosity stabilized at 120,000 mPa·s. The silver paste of Example 2 was obtained by defoaming, grinding and curing in the same manner as in Example 1.

[0082] Example 3:

[0083] In step S1, 100 parts by mass of silver lactate were used as the silver source material, and the pH of the alkaline solution was adjusted to 10.5 with ammonia. The remaining conditions were the same as in Example 1, resulting in modified silver oxide active powder with a large number of hydroxyl groups on the surface. In step S2, 15 parts by mass of ethylene glycol and 15 parts by mass of triethylene glycol monobutyl ether were used as the polyol solvent, and 22 parts by mass of polyurethane resin were used as the resin. The mixture was stirred at 70°C for 60 min to obtain a resin solution. 3 parts by mass of triethanolamine were used as the ligand to give the resin phase stronger metal ion complexing and buffering capabilities. 3 parts by mass of organobentonite were used as the thixotropic agent, 1 part by mass of polycarboxylate dispersant was used as the dispersant, and 3 parts by mass of nano-silica and 4 parts by mass of nano-alumina were used as the inorganic fillers. After dispersion at 1000 rpm for 40 min, the mixture was cooled and degassed to obtain a premixed solution. In step S3, the organic silver complex source was 30 parts by mass of silver β-hydroxybutyrate, the organic solvent was 30 parts by mass of dipropylene glycol methyl ether and 310 parts by mass of polyglycerol-, and the main ligand was 6 parts by mass of N-hydroxyethyl ethylenediamine triacetic acid. The mixture was stirred at 40°C for 40 min to obtain a primary silver complex solution. The auxiliary ligand was 3 parts by mass of sodium polyglutamate, and the stabilizer was 2 parts by mass of carboxymethyl chitosan. The mixture was stirred at 35°C for 30 min to obtain a highly stable silver complex solution. Subsequently, 100 parts by mass of the premixed solution and 45 parts by mass of the silver complex solution were mixed at 25°C, heated to 75°C and stirred for 30 min. Then, 320 parts by mass of modified silver oxide active powder were added in three portions at 40°C. The initial viscosity was 180,000 mPa·s, requiring no adjustment with thixotropic agents or polyols. After degassing for 20 min and three rounds of grinding, the fineness was controlled at 6 μm, and the mixture was cured for 24 h to obtain the silver paste of Example 3.

[0084] Comparative Example 1:

[0085] The silver source system in Example 1 was modified only, and instead of using a silver complex solution, modified silver oxide active powder was used entirely as the silver source to verify the role of the liquid-phase silver complexation pathway. Specifically, steps S1 and S2 were exactly the same as in Example 1. In step S3, the silver complex solution was no longer prepared. Instead, 100 parts by mass of the premixed liquid were directly stirred at 300 rpm at 25°C. Then, 340 parts by mass of modified silver oxide active powder were added in three batches at 40°C, with stirring for 15 min after each addition, to obtain a silver paste intermediate with a viscosity of approximately 160,000 mPa·s. Subsequently, degassing, grinding, and curing were performed according to the same steps to obtain the silver paste of Comparative Example 1.

[0086] Comparative Example 2:

[0087] The modification of silver oxide with long-chain alcohols in step S1 of Example 1 was changed, without adding any long-chain alcohol solvents for surface modification, to verify the effect of long-chain alcohol-modified silver oxide active powder. Specifically, the silver source material, alkaline solution, and reaction conditions were the same as in Example 1. After obtaining wet silver oxide precipitate, the mixing modification with dodecanol in step S1.3 was omitted. Instead, the silver oxide was directly vacuum dried at 50°C for 4 hours, then pulverized and sieved to obtain unmodified silver oxide powder. The preparation steps of steps S2 and the silver complex were exactly the same as in Example 1. In step S3, 100 parts by mass of the premixed solution were mixed with 40 parts by mass of the silver complex solution formulated in Example 1, and 300 parts by mass of unmodified silver oxide powder were added in three portions at 40°C to obtain a silver paste intermediate with a viscosity of approximately 190,000 mPa·s. The grinding and curing conditions were the same as in Example 1 to obtain the silver paste of Comparative Example 2.

[0088] Sample preparation and curing conditions:

[0089] The power amplifier silver paste prepared in each embodiment and comparative example was used to prepare test samples. For electrical and thermal performance tests, the baking conditions for the samples were the same as those for shear strength testing. The silver paste was printed onto the pad area of ​​a 10mm × 10mm gold-plated copper substrate or ceramic substrate using stencil printing. The wet film thickness was controlled at 40 ± 5 μm. After preheating at 100℃ for 1–3 min, the temperature was increased to 200℃ ± 10℃ and held at that temperature in air for 10 min. After curing, the sample was cooled to room temperature in the oven to obtain a cured solder layer sample. For shear strength and chip thermal resistance tests, a 3mm × 3mm power amplifier chip was mounted on the above silver paste pattern with a mounting pressure controlled at approximately 100g. It was then cured according to the same temperature regime to obtain a chip-mounted structure sample. For tests such as fineness and dispersion uniformity, the uncured silver paste or a wet film coated on a glass slide was used as the sample.

[0090] Test procedure for silver powder dispersion uniformity (%RSD):

[0091] The uniformity of silver powder dispersion was characterized by sampling silver paste laterally before curing to detect fluctuations in silver content. Specifically, silver paste was uniformly applied to a clean glass slide using a scraper, with a wet film thickness of approximately 100 μm. After standing for 10 minutes to eliminate leveling effects, the same film was uniformly divided laterally into 10 sampling areas, each approximately 5 mm × 5 mm. Silver paste was scraped from each area using a scraper, and the mass of each sample was accurately weighed. A predetermined volume of nitric acid-sodium chloride mixed solution was added to completely dissolve the silver powder. The mass concentration of silver in each sample solution was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). The average and standard deviation of the silver content in the 10 areas were calculated. The relative standard deviation (%RSD) of the dispersion uniformity was obtained by dividing the standard deviation by the average and multiplying by 100%. A smaller %RSD value indicates a more uniform spatial distribution of silver powder in the silver paste.

[0092] The initial viscosity test procedure:

[0093] The initial viscosity of the silver paste was determined using a rotational rheometer. Specifically, the prepared silver paste was placed in a coaxial cylindrical measuring system with a thermostatic jacket and kept at 25℃±0.5℃ for 10 minutes to allow the silver paste to reach thermal equilibrium. Then, a set shear rate of 10s was applied. - ¹ Perform a shear test and record the apparent viscosity of the system after stabilization as the initial viscosity of this batch of silver paste. To improve repeatability, each sample should be measured at least three times, and the average value should be used as the final reported value.

[0094] The fineness testing process:

[0095] The fineness of the silver paste was determined using a scraper fineness meter. A suitable amount of silver paste was placed at the starting point of the deep groove on a standard scraper fineness meter. A metal scraper was used to scrape along the scale at a constant speed, filling the groove and gradually thinning the paste. The scale position where the scraped surface showed continuous, obvious coarse-grained scratches was observed. The scale value at which continuous, obvious coarse-grained scratches first appeared was the fineness value of the silver paste. Each sample was measured three times, and the average value was taken as the fineness test result.

[0096] The testing process for the sintering neck formation temperature:

[0097] The sintering neck formation temperature was obtained through in-situ resistance-temperature curve determination. First, silver paste was printed between two parallel copper electrodes with a spacing of 1 mm, approximately 3 mm wide. Pre-curing was performed at a lower temperature (e.g., 120 °C) according to the aforementioned process to remove most of the organic solvent without complete sintering. Subsequently, the substrate with the silver paste pattern was placed on a controlled-heating stage, connected to a four-probe or two-probe resistance testing system, and heated from room temperature to 300 °C at a rate of 5 °C / min under a nitrogen protective atmosphere. The resistance of the silver paste bridging region as a function of temperature was continuously recorded. When the resistance showed a significant drop and stabilized with increasing temperature, the temperature range around this inflection point was defined as the sintering neck formation temperature range, i.e., the temperature at which an effective conductive neck begins to form between the silver particles and gradually becomes connected.

[0098] Contact resistance testing procedure:

[0099] Contact resistance was measured using the characteristic resistance of the cured silver solder layer. Silver paste was printed and cured onto a gold-plated copper substrate according to the application process, forming a solder layer pattern with an area of ​​5mm × 5mm and a thickness of approximately 40μm. A thin layer of copper was then electroplated onto the solder layer surface, or a gold layer was used directly as the top electrode. Using a four-terminal Kelvin clamp and a precision DC resistance meter, the voltage drop across the solder layer area was measured under a specified test current (e.g., 100mA). The contact resistance per unit area was then calculated, expressed in mΩ·cm. 2 This indicates that, to eliminate the influence of sample size deviations, at least three substrates were prepared for each formulation, and the average value was taken.

[0100] The process of testing the thermal resistance of a chip:

[0101] Chip thermal resistance was characterized using a steady-state thermal resistance testing scheme for a surface-mount structure. First, standard power amplifier chips were mounted onto copper-ceramic composite substrates with good heat dissipation capabilities using various silver paste formulations, following the aforementioned method, while maintaining consistent curing conditions. Subsequently, a constant DC power was applied to the power amplifier chips at a constant ambient temperature. The chip junction temperature was measured using a built-in junction temperature monitoring diode or an infrared temperature measurement system, and the temperature of the substrate's bottom surface was measured using a thermocouple pre-installed on the substrate's back side. The temperature difference between the junction and substrate was calculated, and the chip-substrate thermal resistance value was derived based on the input power, expressed in K / W.

[0102] The procedure for testing 90° shear strength:

[0103] Shear strength was tested using a dedicated chip shearing tester. Chip-mounted samples cured according to a standardized process were fixed onto the tester's platform. The blade height was adjusted so that the blade was flush against the bottom side of the chip. A load was applied at a specified shearing rate (e.g., 0.5 mm / min or 1 mm / min) along the bottom of the chip, parallel to the substrate surface. The maximum shear force value when the chip was sheared or peeled from the solder interface was recorded. The 90° shear strength, expressed in MPa, was calculated based on the contact area between the chip and the silver paste. At least five chips were tested for each formulation, and the average value was taken as the shear strength result.

[0104] The experimental data are shown in Table 1.

[0105] Table 1:

[0106]

[0107] As can be seen from the experimental results in Table 1, the silver pastes of Examples 1-3 of this invention are significantly superior to Comparative Examples 1 and 2 in key performance aspects such as silver powder dispersion uniformity, initial viscosity control, fineness, sintering neck formation temperature, contact resistance, thermal resistance, and shear strength. The overall performance exhibits the synergistic enhancement effect unique to the preparation process of this invention. Firstly, regarding the silver powder dispersion uniformity, the %RSD of the examples is only about 4-5, while that of the comparative examples is as high as 14-18. This indicates that the silver complex solution path and the long-chain alcohol-modified silver oxide powder in the examples significantly reduce the agglomeration behavior between powder particles, resulting in a more uniform dispersion of silver powder in the polyol-resin system. In particular, the silver complex solution forms a liquid-phase precursor after the premix is ​​added, allowing the silver source to be uniformly distributed in a molecular or coordinated ion state initially. During the subsequent heating stage, it decomposes into silver monomers, which helps fill the gaps between powder particles, thereby reducing the overall dispersion of the powder and improving the rheological stability of the system.

[0108] Secondly, the initial viscosity shows that the examples all maintained a controllable range of 120,000–180,000 mPa·s, while the viscosity of the comparative examples was generally higher, even reaching 190,000 mPa·s. This indicates that the examples established a reasonable balance between the proportion of solid powder and the liquid silver complex. The presence of the silver complex reduces the dependence of the system on all-solid silver powder, thereby avoiding excessive thickening of the system caused by excessive solid content in the comparative examples. In addition, the viscosity window control step S3.2 of this invention can modify the viscosity of the silver paste bidirectionally through thixotropic agents or polyols, making the rheological properties more stable and avoiding the problems of poor stress relaxation and severe printing edge collapse that occurred in the comparative examples.

[0109] Regarding powder fineness, the fineness of the examples was 6–8 μm, while that of the comparative examples was 14–20 μm. This significant difference in fineness indicates that the powder in the example systems was easier to disperse. This is partly due to the reduced surface energy of the long-chain alcohol-modified silver oxide powder, making it easier for the powder to be coated by the dispersant; and partly due to the fact that the nano-silver particles generated during the thermal decomposition of the silver complex can fill the gaps between solid particles, resulting in a finer and more uniform overall particle distribution. Since the comparative examples did not use the silver complex pathway or undergo long-chain alcohol modification, their silver oxide powder particles lacked surface wetting and dispersion assistance, making them more difficult to break during the grinding stage, resulting in a significantly higher fineness.

[0110] As can be seen from the sintering neck formation temperature, the sintering initiation temperature of the embodiments is 160-170°C, while that of the comparative examples all exceeds 200°C. The silver complex solution of the present invention can decompose into highly active silver monomers in the range of 150-180°C, making it easier for silver particles to form initial sintering necks, significantly reducing the low-temperature sintering threshold of silver paste; at the same time, the long-chain alcohol modified powder has higher surface wettability, enabling it to reach the particle contact-rearrangement-fusion state earlier during the heating process, while the comparative examples, due to the higher surface polarity of the particles and the greater interaction forces, make it difficult for the particles to form effective contact, so the sintering neck formation temperature is generally higher, showing obvious low-temperature sintering defects.

[0111] Regarding contact resistance, the contact resistance in this embodiment is only 0.35–0.41 mΩ·cm. 2 The comparative ratio reached 0.87–1.02 mΩ·cm. 2 The low contact resistance of the example lies in the formation of a denser and more continuous silver conductive network after sintering. In particular, the precipitation of nanoscale silver monomers from the complex effectively fills the micro-defects between silver powder particles, making the current path more continuous and thus significantly reducing the interface resistance. In contrast, the comparative example relies entirely on large-particle silver oxide powder, which is difficult to densify fully during sintering. There are more micropores and interface defects between the particles, resulting in higher contact resistance.

[0112] Regarding chip thermal resistance, the embodiments all showed resistance below 1.31 K / W, while the comparative examples reached 2.05–2.21 K / W, indicating that the silver paste of the present invention has superior thermal conductivity. Because the silver complex used in the embodiments forms a denser and more uniform silver network during sintering, its internal heat diffusion path is more continuous, thus facilitating the rapid conduction of heat generated by the chip to the substrate. Furthermore, the long-chain alcohol surface modification in the present invention improves the wettability of the silver paste to ceramic or metal pads, further reducing interfacial thermal resistance; while the comparative examples, lacking this mechanism, have higher porosity within their silver layers and lower interfacial bonding ability, leading to an increase in overall thermal resistance.

[0113] Finally, regarding the 90° shear strength, the shear strength of the embodiments was 30–34 MPa, while that of the comparative examples was only 15–18 MPa, showing a nearly double difference. The high shear strength stems not only from the denser silver layer structure but also from the enhanced interfacial bonding ability. The long-chain alcohol-modified silver oxide powder of this invention exhibits better wetting and spreading properties, enabling a more uniform interfacial contact between the sintered layer and the substrate; simultaneously, the silver monomers generated by the liquid-phase silver complex can deeply fill the interfacial micropores, thereby significantly improving the interfacial bonding strength. The comparative examples, due to poor powder dispersion and insufficient densification, exhibited a clear interfacial failure mode, resulting in significantly lower shear strength.

[0114] In summary, the data presented in Table 1 fully demonstrates the comprehensive advantages of the process of the present invention in terms of powder dispersion, low-temperature sintering, electrical conductivity, thermal conductivity and mechanical reliability, which is a technological improvement that cannot be achieved by the traditional silver paste system.

[0115] This invention proposes a silver paste for power amplifiers, which is prepared by a method for making silver paste for power amplifiers.

[0116] The steps for using silver paste for power amplifiers include:

[0117] The metal or ceramic substrate to be packaged is cleaned. The dispensing pressure, speed or stencil thickness is set according to the chip area and soldering thickness requirements. The dispensing or printing operation is completed in an environment of 20-30℃, so that the silver paste for the power amplifier is uniformly deposited on the pad area of ​​the metal or ceramic substrate, and the film thickness is controlled within the range of 20-60μm.

[0118] Cleaning the metal or ceramic substrate effectively removes adsorbed oil, oxides, and particulate contaminants, restoring the substrate surface to a high level of cleanliness. The surface energy of the clean substrate surface is significantly increased, enhancing its wettability with the resin phase, polyol solvent phase, and silver complex system in the silver paste, thereby improving the uniformity of the silver paste's spread on the substrate surface. With proper settings of pressure, speed, and screen thickness during dispensing or screen printing, the thixotropic network in the silver paste undergoes instantaneous structural breakage under shear force, resulting in excellent shear-thinning properties. This allows the system to pass smoothly through the mesh and form a uniform and continuous film in the pad area. Controlling the film thickness between 20 and 60 micrometers helps to simultaneously ensure the thermal conductivity of the cured silver paste and the stability of chip mounting, providing adequate material space for the subsequent silver deposition reaction. This step ensures that the cured silver layer has a uniform initial geometry, which is crucial for the thermal resistance and mechanical reliability of the power amplifier solder layer.

[0119] Before the silver paste for power amplifiers has cured, accurately mount the power amplifier chip onto the patterned area of ​​the silver paste for power amplifiers. Apply mounting pressure of 50-200g by light pressure or by applying the mounting head. After mounting, let it stand for 1-3 minutes to obtain the mounted substrate.

[0120] Applying moderate pressure to mount the chip before the silver paste hardens ensures thorough microscopic adhesion between the chip's back side and the silver paste, eliminating air gaps at the interface and reducing porosity. The applied light pressure promotes the flow of the continuous resin phase within the silver paste, causing the modified silver oxide active powder and silver complex solution to rearrange under chip pressure, forming a more uniform solid-liquid composite structure. Since the silver complex system remains stable at room temperature and does not prematurely reduce, premature solidification of the silver powder during mounting prevents interface roughness or uneven support. The subsequent settling process allows the thixotropic network to re-establish itself, ensuring the chip remains stably supported and preventing displacement due to excessive silver paste flow. This step ensures chip mounting accuracy, increases the effective contact area between the silver paste and the chip, and lays the foundation for a high-quality thermally conductive solder layer.

[0121] Transfer the mounted substrate to an oven and preheat it at 60–150°C for 30–60 minutes. Then, further increase the temperature to 180–250°C and cure it in an air, nitrogen, or micro-oxygen environment for 60–90 minutes. After curing, cool it to below 40°C to complete the soldering.

[0122] Preheating allows some low-boiling-point polyols in the silver paste system to slowly evaporate, while simultaneously promoting the primary desorption of auxiliary ligands in the silver complex. This transforms the silver complex from a highly stable state to an activated state, creating the reaction prerequisites for the subsequent liquid-phase silver formation pathway. This temperature range does not disrupt the thixotropic network in the silver paste, allowing it to maintain a stable shape without collapse. The modified silver oxide active powder remains structurally stable during this stage, and its long-chain alcohol coating layer remains intact, ensuring the solid-phase pathway is in a reactive state. The preheating process gradually releases internal stress, reducing the probability of interface cracking after curing. The main and auxiliary ligands in the silver complex solution desorb sequentially according to the temperature gradient, allowing silver ions to gradually transform into metallic silver monomers at a controllable rate, and nucleating in situ to form nanoscale silver-filled structures. Simultaneously, the polyol solvent undergoes a mild reduction at high temperatures, causing the silver generated in the liquid-phase pathway to bond with the silver generated from the reduction of solid-phase silver oxide, forming a continuous and dense silver network layer. The atmosphere (air, nitrogen, or micro-oxygen) during the curing stage avoids severe oxidation, ensuring uniform formation of the silver layer under chemical conditions. As the silver network densifies, the thermal conductivity and electrical conductivity of the silver layer continuously improve, ultimately forming a solder layer capable of efficiently conducting chip heat and current load. After curing, controlled cooling to below 40 degrees Celsius prevents interfacial thermal stress concentration caused by rapid cooling. The stable cooling process maintains uniform grain size in the cured silver network structure and allows the resin phase and residual ligands to solidify or cross-link during the cooling stage. This process improves the mechanical stability of the silver layer, reduces the risk of interfacial delamination, and significantly enhances the reliability of the power amplifier under thermal cycling conditions. The cooled solder layer has low porosity and good interfacial bonding, achieving a combination of high thermal conductivity, high mechanical strength, and low thermal resistance, meeting the long-term stable operation requirements of high power density circuits.

[0123] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing silver paste for power amplifiers, characterized in that the steps include... include: S1. Dissolve the silver source material in deionized water to obtain a silver source solution, add an alkaline solution dropwise, stir to react, separate to obtain silver oxide precipitate, add the silver oxide precipitate to a long-chain alcohol solvent, and sieve to obtain modified silver oxide active powder. S2 includes: S2.1 Mix the polyol solvent with the resin, start stirring at 20-30°C and slowly raise the temperature to 60-80°C, stir at 300-500 rpm for 30-60 min to obtain the resin solution; S2.

2. Under the condition of maintaining the resin solution at 60-80℃ and stirring at 300-500rpm, add the complexing agent and continue stirring for 20-40min to obtain a resin solution containing the complexing agent. S2.3 Functional additives include thixotropic agents, dispersants, and inorganic fillers. Thixotropic agents, dispersants, and inorganic fillers are added sequentially to the resin solution containing the coordinating agent. The stirring speed is adjusted to 800-1500 rpm and dispersed for 20-40 minutes to obtain a polyol resin mixture. S2.

4. Stop heating and cool the polyol resin mixture to 20-30°C. Maintain this temperature for 10-20 minutes while stirring at 200-300 rpm. 0.06~ Vacuum degassing at 0.09 MPa for 10–20 min, then stop stirring and let stand to obtain the premixed solution; In step S2, the polyol solvent includes at least one of ethylene glycol, diethylene glycol, and triethylene glycol monobutyl ether; the resin includes at least one of acrylic resin, polyether modified resin, and polyurethane resin; the ligand includes at least one of citric acid, tartaric acid, and triethanolamine; the thixotropic agent includes at least one of fumed silica, polyamide wax, and organobentonite; the dispersant includes at least one of polycarboxylate dispersant and sulfonate dispersant; and the inorganic filler includes at least one of nano silica, nano alumina, and nano zirconium oxide. S3. Mix the premixed solution and the silver complex solution, add the modified silver oxide active powder, heat and stir to obtain silver paste for power amplifiers.

2. The method for preparing silver paste for power amplifiers according to claim 1, characterized in that, Step S1 includes: S1.1 Add the silver source material to a predetermined volume of deionized water and magnetically stir at 300-500 rpm for 10-20 minutes at 20-30°C to obtain a silver source solution. S1.

2. While maintaining stirring, add the alkaline solution dropwise to the silver source solution at a rate of 2-5 mL / min until the pH of the silver source solution reaches 10-12, and continue stirring for 30-60 min. Separate the wet silver oxide precipitate by filtration. S1.3 Add the wet silver oxide precipitate to a long-chain alcohol solvent at a solid-liquid ratio of 1:(3-8), and stir at 300-600 rpm for 30-90 min at 50-80℃ to obtain silver oxide alcohol slurry; S1.

4. The silver oxide alcohol slurry is separated by suction filtration and then vacuum dried at 40-60℃ for 2-6 hours. After pulverization and sieving, modified silver oxide active powder is obtained.

3. A method for preparing silver paste for power amplifiers according to claim 1 or 2, characterized in that, In step S1, the silver source material includes at least one of silver nitrate, silver acetate, and silver lactate; the alkaline solution includes at least one of sodium hydroxide solution, potassium hydroxide solution, and ammonia water; and the long-chain alcohol solvent includes at least one of dodecanol, tetradecanol, and hexadecylol.

4. The method for preparing silver paste for power amplifiers according to claim 1, characterized in that, Step S3 includes: S3.1 Add the premixed solution to the reactor and start stirring at 200-400 rpm at 20-30℃. Add the silver complex solution to the premixed solution at a constant flow rate, controlling the addition time of the silver complex solution to 10-30 min. Continue stirring at 60-80℃ for 20-40 min. S3.2 Adjust the stirring temperature to 30-50℃ and maintain the stirring speed at 300-600 rpm. Add the modified silver oxide active powder in batches, stirring for 10-20 minutes after each addition, until all the powder is added and continue stirring for 20-40 minutes to obtain the silver paste intermediate. S3.3 Cool the silver paste intermediate to 20-30°C, and... 0.06~ Vacuum degassing at 0.09 MPa for 10–30 min, followed by 1–3 rounds of grinding to control the fineness to no more than 10 μm. After grinding, the paste is placed in a sealed container and cured at 5–25 °C for 12–48 h to obtain silver paste for power amplifiers.

5. The method for preparing silver paste for power amplifiers according to claim 4, characterized in that, Step S3.2 further includes adjusting the viscosity of the silver paste intermediate to 80,000–250,000 mPa·s. If the viscosity of the silver paste intermediate is less than 80,000 mPa·s, thixotropic agent or resin is added in several batches to increase the viscosity. The amount of thixotropic agent or resin added each time is 0.1–0.5% of the total mass of the silver paste intermediate. After each addition, the mixture is stirred at 200–400 rpm for 10–20 min. If the viscosity of the silver paste intermediate is greater than 250,000 mPa·s, polyol solvent accounting for 1–5% of the total mass of the silver paste intermediate is added in several batches. After each addition, the mixture is stirred at 200–400 rpm for 5–10 min.

6. The method for preparing silver paste for power amplifiers according to claim 4, characterized in that, Step S3 also includes: The organic complexed silver source is mixed with an organic solvent and stirred at 300-500 rpm for 10-20 min at 20-30°C to obtain an organic silver salt solution. The main ligand is added and the mixture is stirred at 30-40°C for another 20-40 min to obtain a primary silver complex solution. The primary silver complex solution was maintained at 30–40°C and 300–500 rpm under stirring conditions. An auxiliary ligand was added, and the addition time was controlled to be 10–20 min. After the addition was completed, stirring was continued for 20–40 min to obtain a graded silver complex solution. While maintaining a stirring temperature of 30–40°C, a stabilizer is added to the fractionated silver complex solution at a concentration of 0.1–3% of the total mass of the fractionated silver complex solution. The mixture is then stirred at 300–500 rpm for 20–30 minutes to obtain the silver complex solution.

7. The method for preparing silver paste for power amplifiers according to claim 6, characterized in that, Organic complexed silver sources include at least one of silver gluconate, silver citrate, and silver β-hydroxybutyrate. The organic solvent includes at least one of triethylene glycol monobutyl ether, dipropylene glycol methyl ether, and polyglycerol-3; the main ligand includes at least one of tris(hydroxymethyl)aminomethane, polyglycerol-4, and N-hydroxyethylethylenediaminetriacetic acid; the auxiliary ligand includes at least one of dopamine, inositol hexaphosphate, and sodium polyglutamate; and the stabilizer includes at least one of tris(2-ethylhexyl) phosphite, polyvinylpyrrolidone, and carboxymethyl chitosan.

8. A silver paste for a power amplifier, characterized in that, It is prepared by a method for preparing silver paste for power amplifiers as described in any one of claims 1-7; The steps for using silver paste for power amplifiers include: The metal or ceramic substrate to be packaged is cleaned. The dispensing pressure, speed or stencil thickness is set according to the chip area and soldering thickness requirements. The dispensing or printing operation is completed in an environment of 20-30℃, so that the silver paste for the power amplifier is uniformly deposited on the pad area of ​​the metal or ceramic substrate, and the film thickness is controlled within the range of 20-60μm. Before the silver paste for power amplifiers has cured, accurately mount the power amplifier chip onto the patterned area of ​​the silver paste for power amplifiers. Apply mounting pressure of 50-200g by light pressure or mounting head. After mounting, let stand for 1-3 minutes to obtain the mounted substrate. Transfer the mounted substrate to an oven and preheat it at 60–150°C for 30–60 minutes. Then, further increase the temperature to 180–250°C and cure it in an air, nitrogen, or micro-oxygen environment for 60–90 minutes. After curing, cool it to below 40°C to complete the soldering.

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